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Yes if you changed your prefex ending you would MOST likely suffer some ranking problems and then it might be a crap shoot if all of them come back or not.
Then you will have to 301 every page to the exact url in the new prefex.
It's quite reasonable to create a new page title, meta description etc automatically (I include these options as fields in my page-source database).
You MAY lose ranking. But then, google drops ranking and SERPs all the time for no reason at all that anyone can discern - read WebmasterWorld!
One thing I very strongly advise NOT doing: a customer of mine, against my advice, gave his new pages differnt names, so there was no correspondence between old and new. It was impossible to auto-redirect at all, even if the page content was the same (which mostly it wasn't). As far as I know his site still hasn't recovered 18 months later. Some people don't listen. :(
A better solution may be to use an Apache server that allows you to rename the file extensions on the fly. Alternatively, if you have control over your server you can specify that .html (whatever) is really ASP and should be parsed as such. IIS servers aren't particularly clever but that switch is possible.
My site has gone from wordpress (because it has previously been a static section on a wordpress blog) to a custom script to modx without the urls changing.
IMHO any system that does not let you configure what the URLs look like sucks. What happens if it does if it work out and you move again to another platform that insists on its own extension? Two lots of redirects?
As has been said above, changing urls is something to be very seriously considered - the older the site, the less you want to be considering this. Do a site search and check the Hot Topics on this subject as there are some very useful posts explaining the pros and cons.
If you move to another server it is likely you will move to a similar processor (php to php) so the file extensions will remain the same, and that includes htm/l if it was built for, say, PHP with that extension. The primary problem, I would have thought, would arise where the site was never built using a dynamic processor: ie pure html code. In that case you'd be best off changing the processor's default file extension.
fishfinger - you don't need rewrite to process htm/l extensions on ASP. The switch is in IIS under Configuration, where you can specify which DLL processes which file extension. This is on a site basis, so any site on a virtual server can be handled that way.
If your web host isn't willing to change that it's unlikely they will install isapi_rewrite, which requires more work and more money.
Even on the last static site I had (although the static pages were generated by a script) I hid the extensions.
There is one other possibility, but I've not yet been able to play with it. That would be Microsoft's URL Rewrite Module for IIS7 [iis.net] (if you are up to version 7!). WARNING - that link causes my Opera browser to crash.
It's hopeful at least. Note that the url for that page itself is extensionless (http://www.iis.net/extensions/URLRewrite} - along with all the rest of the links on the page.
However, whilst some sites make the change over ok, most get a smack from Google of some description whilst the new site gets re-indexed and settles down.
If you are real unlucky as we were on one of our sites (read my thread page rank 6 to zero in the members area) and re-directs dont get set up properly, you can have a real horror story on your hands and it can take that bit longer for it to sort itself out and you need to be onto it asap - However, it will sort itself out in the end.
At the end of the day i think its always worth improving user experience for the long term, but a long term view you do need to take.
If you depend on that level of traffic or cant afford to weather a potential storm if it falls back for a short period then you need to think hard about the change over. This is why some webmasters just wont change a thing and wont risk it.